Kosaraju Algorithm for SCCs.

Graph/Kosaraju.cpp

@@ -0,0 +1,103 @@
+/* Implementation of Kosaraju's Algorithm to find out the strongly connected components (SCCs) in a graph. 
+   Author:Anirban166
+*/
+
+#include<iostream>
+#include<vector>
+
+using namespace std;
+
+void print(vector<int> a[],int V)
+{
+    for(int i=0;i<V;i++)
+    {
+        if(!a[i].empty())
+            cout<<"i="<<i<<"-->";
+        for(int j=0;j<a[i].size();j++)
+            cout<<a[i][j]<<" ";
+        if(!a[i].empty())
+            cout<<endl;
+    }
+}
+void calculate_time(int v,stack<int> &st,bool vis[],vector<int> adj[])
+{
+    vis[v]=true;
+    for(auto i=adj[v].begin();i!=adj[v].end();i++)
+    {
+        if(vis[*i]==false)
+            calculate_time(*i,st,vis,adj);
+    }
+    st.push(v);
+}
+void dfs(int v,bool vis[],vector<int> grev[])
+{
+    vis[v]=true;
+    // cout<<v<<" ";
+    for(auto i=grev[v].begin();i!=grev[v].end();i++)
+    {
+        if(vis[*i]==false)
+            dfs(*i,vis,grev);
+    }
+}
+int kosaraju(int V, vector<int> adj[])
+{
+    // print(adj,V);
+    bool vis[V]={};
+    stack<int> st;
+    for(int v=0;v<V;v++)
+    {
+        if(vis[v]==false)
+            calculate_time(v,st,vis,adj);
+    }
+    // cout<<"stack_size="<<st.size()<<endl;
+    //making new graph (grev) with reverse edges as in adj[]
+    vector<int> grev[V];
+    for(int i=0;i<V+1;i++)
+    {
+        for(auto j=adj[i].begin();j!=adj[i].end();j++)
+        {
+            grev[*j].push_back(i);
+        }
+    }
+    // cout<<"grev="<<endl;
+    // print(grev,V);
+    //reinitialise visited to 0
+    for(int i=0;i<V;i++)
+        vis[i]=false;
+    int cnt_scc=0;
+    while(!st.empty())
+    {
+        int t=st.top();
+        st.pop();
+        if(vis[t]==false)
+        {
+            dfs(t,vis,grev);
+            cnt_scc++;
+        }
+    }
+    // cout<<"cnt_scc="<<cnt_scc<<endl;
+    return cnt_scc;
+
+}
+//All critical/corner cases have been taken care of.
+//Input your required values: (not hardcoded)
+int main()
+{
+    int t;
+    cin>>t; 
+    while(t--)
+    {
+        int a,b ;
+   	    cin>>a>>b;
+   	    int m,n;
+   	    vector<int> adj[a+1];
+        for(int i=0;i<b;i++)
+        {
+            cin>>m>>n;
+            adj[m].push_back(n);
+        }
+        // exit(0);
+        cout<<kosaraju(a, adj)<<endl;
+    }
+    return 0;
+}